1. Field of the Invention
The present invention relates to a sheet feeding apparatus and an image forming apparatus, and more specifically to control on timing for starting to feed a sheet.
2. Description of the Related Art
Hitherto, an image forming apparatus such as a printer, a copier, a facsimile or the like is configured to deliver an uppermost sheet within a bundle of sheets stacked in a sheet feed cassette one by one and then to convey the sheet by a sheet feeding apparatus so that the sheet arrives at an image forming portion at a predetermined timing. Here, the image forming portion is configured to form an image on an image carrier by receiving a signal from a control portion. Then, the image on the image carrier is synchronized with the sheet conveying timing to transfer the image to a predetermined position on the sheet. The image is fixed on the sheet under heat and pressure applied in a fixing portion, and the sheet is discharged by a discharging portion to a sheet discharge portion. In a case when such image forming operation is conducted consecutively, the sheets are conveyed at predetermined intervals (inter-sheet intervals) and the image forming portion also forms images at the same intervals to print consecutively.
Here, a conventional sheet feeding apparatus is provided with conveyance sensors detecting the conveyance of the sheet at main parts of a sheet conveying path. Then, a control is made such that such conveying operation is normally conducted by detecting a conveyance state of the sheet by detecting an arrival timing of an downstream side in a sheet conveying direction (front end) of the sheet and a passage timing of a upstream side in the sheet conveying direction (rear end) of the sheet by the conveyance sensors.
As such a conveyance sensor, there is known one including a sensor flag that swings by being in contact with the sheet and a photo sensor for example. This conveyance sensor is configured to output a signal indicating an arrival of the sheet to a control portion such that the sensor flag swung by the sheet being conveyed blocks an optical path of the photo sensor. It is noted that because the sensor flag continues to block the optical path of the photo sensor during when the sensor flag is swung by the sheet, a signal indicating that the sheet exists on the conveyance sensor is inputted to the control portion. When the sheet passes through the sensor flag, the conveyance sensor also outputs a signal indicating that the sheet has passed through because the sensor flag returns to its original position and the optical path of the photo sensor is opened. Thus, the conveyance sensor of this type detects whether or not the sheet exists on the sensor flag by the contact of the sensor flag with the sheet.
The conventional sheet feeding apparatus also includes a separating portion composed of a conveying roller formed of a high-friction member such as rubber and a separating member configured to bias the sheet from a back surface thereof to apply a conveyance load acting in a direction opposite from a sheet conveying direction. This configuration prevents double-feeding, i.e., a succeeding sheet from being delivered from the sheet feed cassette while partly overlapping with a preceding sheet, by arranging such that the sheet delivered from the sheet feed cassette passes through the separating portion. However, if an operation time becomes long, it becomes difficult to completely prevent the double-feeding due to a drop of a friction force of the separating member caused by its wear and adhesion of paper dust, to increase of a friction force between sheets, and others.
Then, the conventional sheet feeding apparatus is provided with a double-feeding detecting portion configured to detect double-feeding of sheets along a sheet conveying path. As such a double-feeding detecting portion, there is one configured to detect the double-feeding by judging whether one sheet or two or more sheets are passing from changes of transmittance by using a photo diode as disclosed in Japanese Patent Application Laid-open No. H07-121079. The sheet feeding apparatus using such double-feeding detecting portion is configured, when the double-feeding is detected, to measure a shift amount between a front end of a preceding sheet and a doubly fed sheet and to delay a next sheet feed operation by a time corresponding to the shift amount. Such control makes it possible to continue consecutive image forming operations in which a predetermined sheet interval is maintained even after detecting the double-feeding.
As another double-feeding detecting portion, there is one configured to detect whether no sheet, one sheet or two or more sheets are passing by utilizing such characteristics that frequency of ultrasonic attenuates when the ultrasonic transmits an object as disclosed in Japanese Patent Application Laid-open No. 2007-169044. The sheet feeding apparatus using such double-feeding detecting portion is configured to calculate a shift amount between a front end of a preceding sheet and a front end of a doubly-fed sheet by the ultrasonic and to adjust position of the doubly-fed sheet to toner image position on the image carrier by adjusting a sheet conveying speed. This configuration makes it possible to transfer the toner image on the image carrier to the predetermined position of the sheet even if the sheet is doubly fed. This configuration also makes it possible to continue the image forming operations successively after the double-feeding by maintaining the predetermined inter-sheet intervals by adjusting feed timing of the succeeding sheet.
By the way, even if the shift amount of the doubly-fed sheet is measured at the front end in the sheet conveying direction, there is a case when the shift amount of the doubly-fed sheet changes when the sheet is conveyed. Because the conventional double-feeding detecting portion is configured to detect changes of thickness of sheets, it is unable to judge whether the doubly-fed sheet is being conveyed or is stopped. Therefore, even if the shift amount of the doubly-fed sheet is measured, it is difficult to utilize the measured shift amount to such controls as changes of conveying speed and changes of feed timing of the succeeding sheet.
The doubly-fed sheet is stopped during its conveyance in the following cases. Normally, a conveying roller pair conveying a sheet is composed of a conveying roller formed of rubber or the like rotationally driven and exhibiting a high conveying force, and a driven roller formed of resin and elastically biased to the conveying roller. When doubly-fed sheets are conveyed, while a superordinate preceding sheet in contact with the conveying roller is conveyed downstream by receiving the high conveying force of the conveying roller, a subordinate succeeding sheet in contact with the driven roller is conveyed only by a friction force acting between the preceding sheet and the succeeding sheet.
Due to that, there is a case when a slip is generated between the succeeding sheet and the preceding sheet or the succeeding sheet stops if additional conveyance resistance is applied in passing through a curved sheet conveying path for example. If the succeeding sheet slips here, a shift amount at a rear end required to change a next feed operation increases more than the shift amount measured at the front end. Therefore, if the next feed operation is carried out based on the shift amount measured in terms of the front end, the next feed operation is started in a state in which there is no inter-sheet interval from the rear end of the previous doubly fed succeeding sheet.
In this case, because it is unable to detect a front end of a next sheet, it is unable to match the front end of the sheet with an image in the transfer portion. Still further, if the succeeding sheet stops, it becomes unable to continuously convey the stopped succeeding sheet because the conventional double-feeding sensor is unable to estimate a position where the succeeding sheet has stopped. Thus, the retention of the sheets must be treated as jamming.
Still further, the conventional double-feeding detecting portions detecting changes of thickness of the sheets by the transmittance and the attenuation of ultrasonic are required to adjust double-feeding detecting level corresponding to types of sheets such as thick and thin sheets. As a method for adjusting the detecting level, a user is required to adjust the detecting level by specifying the type of the sheet in starting to print. Then, a control portion judges whether a conveyance is a normal conveyance in which one sheet is conveyed or double-feeding in which two or more sheets are conveyed based on data stored in advance. As another method for adjusting the detecting level, the double-feeding detecting portion is reviewed by a sheet conveyed by a first feed operation after detecting opening/closure of the sheet feeding cassette. Then, based on reviewed information, the control portion discriminates whether a sheet conveyed from a second sheet or after is one sheet or double-feeding of two or more sheets.
However, in the former case, the user who is going to print does not always know the type of the sheet stacked in the sheet feeding cassette even if the type has been changed in a network printer. Therefore, there is a possibility that the user specifies a wrong sheet type and the double-feeding detecting portion judges as double-feeding and shifts to a double-feeding sequence even if the sheet is being normally conveyed. In the latter case, it is unable to discriminate whether the first sheet is being normally conveyed or doubly fed because the review is made by the value of the first time of the feed operation. Therefore, there is a problem that if the first sheet is doubly fed, it is unable to correctly detect double-feeding after that. Still further, it is unable to discriminate between the normal conveyance and the double-feeding because an output of the double-feeding detecting portion varies in conveying sheets in a case where a plurality of types of sheets is mixed. Therefore, in the case where the plurality of types of sheets is mixed, it is unable to convey the sheets consecutively without nullifying the double-feeding detecting portion.